Has anyone done this test? A gravel bike that can accommodate a 2.2 vs a bike that can only accommodate a 47 mm tire. Theoretically, the latter bike frame and fork should be more aero.
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Actually yes I have that data but with different riders⌠the bike with bigger tires was faster CDA wise. Was also a carbon frame vs steel. This would be a really tough metric to nail down and from the testing Iâve done and very hard to predict which one would be faster, purely based on tire clearance.
You donât need a test for that, it is a very simple physics problem.
It sounds like the scenario he is talking about is accelerating from a low-ish climbing speed to a fast speed to respond to attacks, and how a lighter tire will be an advantage.
Letâs break that down- if we compare the kinetic energy of a 29" cylindrical shell with 425g (40mm Reaver) vs 700g (2.4" Aspen ST) at both 10mph and 20mph you get:
At 10mph: 4.25 Joules for the 40, and 7 Joules for the 2.4. (2.75J delta)
At 20mph: 17J for the 40, and 28J for the 2.4. (11J delta)
That means there is an energy difference of 8.25J to increase the rotational kinetic energy of those tires.
You also have the translational kinetic energy which is the same amount as an ideal shell with all the mass at the outside, 8.25J delta. So in total it takes 16.5J extra, or 3.3 watts if the acceleration takes 5 seconds.
Compare that to the power required to accelerate the entire system: for a 180lb mass going from 10 to 20mph it is 2447J, or 489W over 5 seconds.
3.3/489 is 0.6%.
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The thing that made me laugh about the Nathan Haas interview was the confidence with which he proclaimed âthe science is flawedâ. To give a couple more of his quotes about wider/heavier tyres:
âBut the one thing they donât actually take into account is the amount of energy it takes to get that tyre to that speed. So now weâre talking about how many KJoules of energy it takes to accelerate for zero or from 5, 10 kph or up to 30â
âThey take a hell of a lot more time to speed upâ
If I was going to go onto a podcast, listened to by thousands of people, Iâd personally spend a few minutes to check whether it is really is a significant number of KJ to use heavier tyres. Iâd want be sure Iâm not going to say a load of nonsense.
Itâs taken me longer to write this reply than it was to do the calculation. It only needs a high school level understanding of physics to find the equations for energy and then apply them.
For additional mass at the rim or tyre, the total additional energy (linear and rotational energy) is the incremental mass (in kg) multiplied by the speed (in m/s) squared. The effect of the rotating mass is twice that of non-rotating mass, as others have commented already. As @crandallGA said in post #513, the difference in weight between a pair of good gravel tyres and a good pair of MTB tyre is 226-414g. Letâs call it 400g.
How many extra KJ is it to accelerate that 0.4 kg of additional rotating mass from zero up to 30kph? 0.029 KJ extra, thatâs how much. These elite riders burn about 1000 KJ per hour donât they?
Thatâs why I laughed when I heard him talk about all those KJ to accelerate the heavier tyres.
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Looks like we were doing very similar calculations at the same time!
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You guys with your fancy math and formulas are forgetting that it âfeelsâ slower to accelerate a heavier wheel and clearly that trumps science.
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We can even simplify the math further. Acceleration (whether gaining speed or elevation) is linearly proportional to mass.
Letâs say pro rider plus bike minus rims/tires (the spinny bits) is 170lb.
A light rim/tire combo is say 2lbs. Bump up to a MTB tire adds a half pound.
A rolling wheel costs about double in weight if you do the math.
Base weight would be 170 lbs plus 2lb * 2 wheels * 2x penalty = 178 lb equivalent
MTB tire weight would be 170 plus 2.5 * 2 * 2 = 180 lb equivalent
2/178 = ~ 1% penalty on the portion of your energy output doing the acceleration, which doesnât include however much energy youâre expending against rolling resistance and aerodynamic drag. Those increase based on speed, so the faster your effort, the smaller that 1% increase becomes in the big picture. So for the pros that number becomes quite insignificant.
Not really or not necessarily, respectively. 2.2 clearance is significantly more than 47 mm but not really that much in respect to whatâs needed. Most bikes are more restricted in the bottom bracket yoke or for the rear wheel than at the fork anyways. So there will hardly be any difference on the rear end and that part of the bike is less responsible for the total drag of the bike anyways.
Then the fork⌠tire clearance is not really a factor which is responsible for the aerodynamic properties of a fork either. Thatâs more the shape of the fork itself.
What you would get would be a comparison of two gravel bikes. Always nice to have such data but it would not show you what compromise youâd have to make to have a normal gravel bike clearing 2.2 inch instead of 47 mm. Not even if that normal gravel bike already had a bit of aero consideration going on into its design.
Great stuff. Follow-up question if you donât mind. Suppose you can go into the turn at a higher speed with the Aspens due to better grip. At what point does it actually take less energy to accelerate the Aspen vs the Reaver?
Right at 1% faster, so 10.1mph or so! Itâs because the energy costs to accelerate the system are way beyond the savings of rotational inertia.
edit: OK it would be 2% for two tires, so 10.2mph.
This may be getting at the reason why we think lighter setups feel faster even if they are not, because we as humans are terrible at sensing speed but we can feel how much effort we are putting out. So we can actually notice something feel easier to pedal but completely miss the fact that we were carrying 1mph extra speed with a different setup and required less output overall. In conclusion, donât trust feeling for tire testing!
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I had a chance to do a little testing today, mostly just to see if my protocol would get consistent results but I was wanting to know how much harder it is to push my gravel wheels vs. my road wheels. This probably wonât apply anybody elseâŚbut it looks like roughly 3% faster on the road wheels. And this is on smooth pavement so adding âtextureâ would only make the gravel setup look better. Although I would say that it feels a hell of a lot more than 3%! I rode some laps on the gravel wheels, then road, then back to gravel, then back to road. The gravel wheels got faster on their second stintâŚthe road wheels got slower. I donât understand thisâŚbut anywayâŚhere is the dataâŚ
Joe
Road vs. Gravel wheels.pdf (16.6 KB)
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What road wheels and tires and gravel wheels and tires?
This stuff seems so difficult to measure. While the average power is pretty good, what did the graph look like during the interval? On gravel was it difficult to take the same line every time? Was there wind? Did the temperature change that could impact the power meter slightly? Once the tires get warmed up, how does that impact results?
The questions could go on and on⌠which is what makes this so difficult. And what makes those willing to experiment so valuable. I think many just throw numbers out without any form of validation. So I guess this is a long way to say thanks for all the effort and trying to put some kind of data to the hypothesis. Itâs not easy!
OK so donât laugh. Gravel setup is the e thirteen piedmont wheels with a 2.1" thunder burt super ground addix speed rear and 45mm vittoria strada bianchi (the handmade one) in front. I rode this setup in big sugar. Inserts on both ends. On the road setup itâs the bontrager Aeolus pro 3v wheels with a 32mm conti 5000 in back and a pirelli p zero up front, both tubeless with an insert in the rear. So a mishmash but, in general, your typical fast-ish road setup and fast gravel setup.
Joe
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âChallenging at best, deeply problematic at worstâ LOL, I hear you!
This was on new smooth pavement so we didnât have the âline in gravelâ issues but there was some wind and the temp probably varied a little (test took about an hour so maybe not too bad on temp). I havenât seen tires get faster when they warm up but itâs something to watch for (hadnât considered it).
As far as where is the power delivered during the lap, that seems to make surprisingly little difference. And I like time trials so the concept of âpush a little on the hills and back off a little on the descents for a faster time than a steady powerâ is taken as truth. But I havenât been able to prove it matters muchâŚif at all. I think if you run a lap a whole bunch of times going for consistency youâll end up putting down the power similarly run-to-run. I try to keep it steady and keep an eye on ave power as the lap progresses but Iâm sure there is plenty of variability there. Iâm just not convinced either way about it mattering (maybe it doesâŚmaybe not?)
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OMG triggering! You run a lot of laps and look at the data and itâs likeâŚâif we take lap 4 and 9 then these setups are the same but if we look at laps 2 and 7 then A is way faster than Bâ so when you see a youtube video with one trial for each setupâŚeven if they are using power to measure the effortâŚitâs likeâŚwow that told us nothing.
Joe
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Itâs good that youâre aware of that. Iâm sure a lot of people that do testing, targeting a fixed average power per run, donât realise that by pushing a bit harder in the ârightâ places, on the uphills, in an effort to bring the average back up towards the target power, it affects their overall times/average speeds, even though their average powers are spot on their target.
Thereâs a lot of potential pitfalls with testing, but the fairly small variations seen in your lap-to-lap times, and also the stint-to-stint time variation, provide some really good insights into the precision of the test method.
Regarding temperature, I remember than Tom Anhalt derived a relationship between temperature and CRR (see plot in this blog post), albeit it with a fairly limited number of test points. It would be good to know if thereâs any other data out there. Intuitively, it seem right that a warmer tyre would be more supple and have less hysteresis losses (like a squash ball, once it becomes warm).
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Just fitted the new Schwalbe G-One RS 50mm on my Checkpoint SLR 9 (2023 model). Looks like itâs going to be a great tire and fit. Excited to give it a go coming from the older version 45mm.
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Nice, is this tire in the BRR queue for testing?
BRR has performed a CRR at temperature test as well: CRR Temperature Test | Bicycle Rolling Resistance
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